The present disclosure relates to an antenna device.
In recent years, various pieces of electronic apparatus such as mobile phones, tablet terminals, and mobile game machines have come to be equipped with wireless communication functions. Because these pieces of apparatus are used near a human body in a telephone call and data transfer, there has been concern about influence of radio waves on the human body. In Japan, in consideration of this concern, the specific absorption rate (SAR), which is the amount of energy absorbed by the human body, is defined by law about radio waves radiated by mobile phones and so forth.
The SAR is the average of the amount of energy absorbed by the human body of unit mass per unit time, and two criteria, whole-body SAR and local SAR, are defined. The SAR is defined by the following calculation formula.SAR=σ|E|2/ρ
E: electric field intensity [V/M]
σ: conductivity of human body tissue [S/m]
ρ: density of human body tissue [kg/m3]
Regarding the local SAR, a criterion is defined on each country basis. For example, in Japan, the average of energy absorbed per 10 g of human tissue in six minutes is restricted to at most 2 W/kg. In the US, it is restricted to at most 1.6 W/kg per 1 g of human tissue. To meet the SAR criterion in each country, various techniques have been proposed.
One of them is a method in which transmission power from an antenna is lowered when the approximation of a human body to the terminal is detected by using a proximity sensor or the like. However, this method is not preferable because energy radiated from the terminal is reduced and thus the communication performance is sacrificed.
Techniques for reducing the SAR without changing the transmission power have been proposed in Japanese Patent Laid-opens No. 2003-258523, No. 2007-67512, and No. 2002-26627 (Patent Documents 1 to 3). In the techniques described in Patent Documents 1 to 3, the directionality of a radiation pattern from an antenna is aggressively changed to reduce electromagnetic waves radiated in a specific direction in which a human body exists. These techniques are effective in electronic apparatus, such as a mobile phone terminal, used with keeping of a specific positional relationship with a human body in a telephone call.